The Persian Wars were a series of wars between the city-states of Greece and Persia that lasted approximately twenty years, with periodic battles afterwards (Britannica Concise Encyclopedia, 12). This conflict began under the Persian ruler, Darius I, who was forced to end revolts against Persia by many Greek city-states (Burstein, 263). These revolts began in 499 BC and caused Darius I to become irate with Greece (Burstein, 263). It took Darius almost ten years, but in the year 490 BC, he and his Persian army invaded Greece in the Battle of Marathon (Burstein, 263). To Darius' surprise, although he had more soldiers than the Greeks, his army was defeated due to the fact that Greece had more advanced weapons and better trained military personnel (Burstein, 263).
Darius' son, Xerxes I, had better luck at defeating the Greeks at the Battle of Thermopylae (Classical Literature Companion, 3). This battle was fought between Persia and the Spartans (Shek, 264). Although the Spartans were fierce soldiers, they were tricked by one of their own, and were defeated by the Persians in a narrow mountain pass (Shek, 265). Although the Persians gained a triumph in the battle, their victory was short lived. The Greeks did become the ultimate champions in the Persian Wars. They won the Battle of Salamis and the Battle of Plataea (Shek, 265). In the Battle of Salamis, the Greeks sunk most of the Persian fleet; the Persians were forced to go back to Persia (Shek, 265). The final blow for the Persians, at Plataea, was a defeat by many city-states of Greece who had come together to defeat them (Shek, 265).
After the defeat of the Persians, Greece felt empowered. Athens was considered to be the most powerful city-state of Greece because of its powerful navy (Gill, https://ancienthistory.about.com/cs/greecehellas1/a/delianleague.htm). With Athens at the top, Greece joined the Delian League in order to keep Greek city-states together and to prevent Greece from any further defeats (Gill, https://ancienthistory.about.com/cs/greecehellas1/a/delianleague.htm). Before the Persian Wars and the creation of the Delian League, most Greek city-states acted as though they were their own country; this aimed to keep them together as a like people (Gill, https://ancienthistory.about.com/cs/greecehellas1/a/delianleague.htm). The Delian League was successful until 404 BC, when Sparta became paranoid that Athens was too powerful and waged a war against them; this broke the relationship of the Greek city-states (Gill, https://ancienthistory.about.com/cs/greecehellas1/a/delianleague.htm). Greece would spend many more years at war, but this time with itself.